The present invention relates to a crash or collision detection apparatus of an air bag system and, more particularly, to a crash detection apparatus having a diagnostic function used for an air bag system of an automobile.
As a crash detection apparatus of an air bag system in which an automobile crash is detected and an air bag is inflated, there is a known detection apparatus which employs a piezoelectric element making use of a piezoelectric effect or a semiconductor strain gage utilizing piezoelectric resistance. These apparatus are disclosed in U.S. Pat. No. 4,638,179, or in JP A 2-95952, for example. In the former apparatus employing the piezoelectric element, a mass (weight) part is supported by a cantilever made of piezoelectric ceramic, and strain caused in the cantilever by displacement of the mass part due to acceleration is detected through the piezoelectric effect. In the latter detection apparatus, a mass part is supported by the cantilever made of the piezoelectric resistance element, and strain caused in the cantilever in the same manner as mentioned above is detected by the piezoelectric effect.
In this kind of the crash detection apparatus, it is necessary in view of safety to automatically diagnose the crash detection system periodically on whether or not the crash detection apparatus can operate correctly.
In conventional diagnosis of the crash detection apparatus, in case the piezoelectric element is used as a crash sensor, a cantilever of a piezoelectric element is driven by applying electric voltage out of the system and utilizing the piezoelectric effect, and the sensor is diagnosed by the movement of the weight part of the cantilever. However, an electric source of high voltage as a driving voltage is necessary in order to drive the piezoelectric element having the weight. Further, signal source impedance from the piezoelectric element is so high that a charge amplifier of high input impedance is needed as a signal amplifier, which results in being large in scale of diagnostic apparatus.
On the other hand, in case the above-mentioned semiconductor strain gage is used, processing of detection signals is advantageous in that it is difficult to be influenced from externally induced noises and easy to be processed because the signal source impedance is low. However, since there is no means for electrically driving the piezoelectric element for diagnosis, it is necessary to provide a diagnostic means different from the above-mentioned diagnostic means.